JP3873398B2 - Oxygen sensor element and manufacturing method thereof - Google Patents

Description

【００９２】
実施形態例２
本例は，保護層にある細孔を粒径０．１μｍ以下のセラミックの微粒子で充填し，これら微粒子を焼結させて作製した緻密層を有する酸素センサ素子及び本例にかかる緻密層の製造方法について，図７，図８を用いて説明する。
【００９３】
本例にかかる酸素センサ素子は実施形態例１と同様の構造を有しており，コップ型の固体電解質体１０と該固体電解質体１０の表面に設けられた一対の測定電極１１，基準電極とよりなる。
上記測定電極１１は保護層１２により被覆されてなり，かつ上記保護層１２はその表面部分の一部に内部より緻密化した緻密層１３を形成している。
また，上記固体電解質体１０には内室１９が設けてあり，該内室１９には図示を略したヒータが挿入されている。
【００９４】
本例にかかる保護層１２，緻密層１３につき説明する。
本例にかかる保護層１２も実施形態例１と同様にＭｇＯ・Ａｌ₂ Ｏ₃ スピネルよりなる。また，図７に示すごとく，上記保護層１２はプラズマ溶射にて形成された多孔質層である。なお，符号１２９はＭｇＯ・Ａｌ₂ Ｏ₃ スピネル粒子である。
そして，上記保護層１２の表面における細孔は多数の微粒子１３９により閉塞されており，ここに緻密層１３が形成されることとなる。
上記微粒子１３９はＡｌ₂ Ｏ₃ 粒子よりなり，その粒径は０．１μｍ以下である。また，上記緻密層１３の気孔率は３％である。
その他は実施形態例１の酸素センサ素子と同様である。
【００９５】
本例にかかる緻密層の製造方法について以下に説明する。
実施形態例１と同様にして，固体電解質体１０，測定電極１１，保護層１２等を作製する。
次いで，上記保護層１２に対して，アルミナゾル溶液を筆塗りにより塗布した。これにより，上記アルミナゾル中に存在するアルミナの微粒子１３９が上記保護層１２の表面の細孔に充填された。
【００９６】
次いで，上記固体電解質体ごと塗布したアルミナゾル溶液を加熱する。これにより，上記微粒子１３９は互いに焼結し，あるいは微粒子１３９と保護層１２を構成する粒子１２９とが焼結し，緻密層１３を形成した。
【００９７】
次に，本例にかかる酸素センサ素子の性能につき，前述の図５，表２を用いて説明する。
本例にかかる酸素センサ素子について，緻密層１３を設けた位置を表２に示すごとく種々に変化させて製造した試料２１〜２９を準備した。これらの試料の耐久性及び初期応答性について実施形態例１と同様に測定した。なお，同表におけるＦ，Ｇは実施形態例１，表１，図５と同様の意味である。
【００９８】
表２における試料２３〜試料２５，試料２８，２９により，実施形態例１と同様に上記保護層と酸素センサにかかるハウジングとが接触する部分と，酸素センサ素子の先端より１０ｍｍ上方の部分との間に少なくとも緻密層を設けることにより，初期応答性が高く，耐久性の高い酸素センサ素子が得られたことが分かった。また，本例にかかる製造方法においても，実施形態例１と同様に優れた酸素センサ素子を作製できることが分かった。
【００９９】
【表２】

【０１００】
実施形態例３
本例は，図９に示すごとく，ガラス質よりなる緻密層を有する酸素センサ素子について説明するものである。
本例にかかる酸素センサ素子は実施形態例１と同様の構造を有しており，コップ型の固体電解質体１０と該固体電解質体１０の表面に設けられた一対の測定電極１１，基準電極とよりなる。
上記測定電極１１は保護層１２により被覆されてなり，かつ上記保護層１２はその表面部分の一部に内部より緻密化した緻密層１３を形成している。
また，上記固体電解質体１０には内室が設けてあり，該内室には図示を略したヒータが挿入されている。
【０１０１】
上記緻密層について説明する。
本例にかかる緻密層１３はガラス質の耐熱性被膜よりなる。
具体的には，ＭｇＯ及びＡｌ₂ Ｏ₃ の粉末と，硼珪酸ガラス等の低融点ガラスをエチルセルロース等の樹脂を溶かした溶液をスリップ状となし，これを筆塗りによって，上記保護層１２の所望の部分に付着させ，温度８００〜１０００℃で焼き付けることによって形成された被膜よりなる。
【０１０２】
なお，上記温度が８００℃未満である場合は，材料中のガラスが充分溶けず，保護層との付着力が不充分となり，酸素センサ素子の使用中に緻密層が剥離するおそれがある。一方，上記温度が１０００℃より高い場合には，電極が凝集するおそれがある。
【０１０３】
そして，本例にかかる緻密層１３の熱膨張率は保護層と略等しい膨張率を有している。つまり，保護層１２の熱膨張率は７５〜８５×１０^-7／℃，緻密層１３の膨張率は７５〜８５×１０^-7／℃である。
【０１０４】
なお，上記被膜としては，熱膨張率が保護層１２と同一か近似したもの，更に酸素センサ素子の使用温度となる５００〜７００℃程度の温度において軟化しない材料であれば，上述した材料以外の物質で構成されていてもよい。
【０１０５】
次に，本例にかかる酸素センサ素子の性能につき，前述の図５，表３を用いて説明する。
即ち，図５にかかるＦ及びＧが表３に示す値となるような緻密層を設けた酸素センサ素子（試料３１）を準備し，実施形態例１と同様に耐久性と初期応答性について測定した。
その結果，同表より知れるごとく，本例にかかる緻密層を有する酸素センサ素子は優れた耐久性及び初期応答性を有することが分かった。
【０１０６】
【表３】

【０１０７】
実施形態例４
本例は図１０に示すごとく，限界電流式の酸素センサ素子４である。
本例にかかる酸素センサ素子４は，コップ型の固体電解質体１０とその内部に設けた内室１９とよりなり，かつ上記固体電解質体１０の外表面には測定電極４２１及び絶縁層４１１，４１２を，上記内室１９に面する内表面には基準電極１４を設けてなる。
また，上記内室１９には図示を略したヒータが挿入され，上記測定電極４２１にはこれを保護するための保護層４３，４２２が，更にその外側には被毒物トラップ層４２３が設けてある。
【０１０８】
そして，上記保護層４３はＭｇＯ・Ａｌ₂ Ｏ₃ スピネルよりなり，プラズマ溶射にて形成された多孔質層である。そして，上記保護層４３の表面にある緻密層１３は上記多孔質な保護層４３の表面に存在する細孔を塞ぐように形成されており，リン酸アルミニウムを含有している。
また，上記緻密層１３は，実施形態例１に示したごとく，上記保護層４３の表面にリン酸を筆塗りによって塗布し，これを加熱することにより作製する。
その他は実施形態例１と同様である。
また，本例の酸素センサ素子４は実施形態例１と同様の効果を有する。
【図面の簡単な説明】
【図１】実施形態例１にかかる，酸素センサ素子の縦断面説明図。
【図２】実施形態例１にかかる，酸素センサ素子の図１にかかるＡ−Ａ矢視断面図。
【図３】実施形態例１にかかる，酸素センサ素子の要部拡大説明図。
【図４】実施形態例１にかかる，本例にかかる酸素センサ素子を設けた酸素センサの断面説明図。
【図５】実施形態例１にかかる，表１のＦ及びＧを示す説明図。
【図６】実施形態例１にかかる，トラップ層を設けた酸素センサ素子の縦断面説明図。
【図７】実施形態例２にかかる，酸素センサ素子の要部拡大説明図。
【図８】実施形態例２にかかる，保護層と緻密層との説明図。
【図９】実施形態例３にかかる，酸素センサ素子の要部拡大説明図。
【図１０】実施形態例４にかかる，酸素センサ素子の縦断面説明図。
【符号の説明】
１，４．．．酸素センサ素子，
１０．．．固体電解質体，
１０１．．．外表面，
１０９．．．内表面，
１１．．．測定電極，
１２，４３．．．保護層，
１３．．．緻密層，
１４．．．基準電極，
１９１．．．先端部，
１９２．．．基端部，[0001]
【Technical field】
The present invention relates to an oxygen sensor element mainly used for combustion control of an automobile internal combustion engine or the like and a manufacturing method thereof.
[0002]
[Prior art]
In order to detect the oxygen concentration in the exhaust gas of an automobile internal combustion engine, a gas sensor installed in the exhaust system of the internal combustion engine is ZrO.₂For example, an oxygen concentration electromotive force type oxygen sensor element using a solid electrolyte body is well known and put into practical use.
Such a gas sensor includes, for example, a housing and an oxygen sensor element inserted in the housing as shown in FIG.
[0003]
The oxygen sensor element comprises a cup-shaped solid electrolyte body and an inner chamber provided therein, and a measurement electrode is provided on the outer surface of the solid electrolyte body, and a reference electrode is provided on the inner surface facing the inner chamber. It is provided. The measurement electrode is provided with a protective layer for protecting it. A heater is inserted into the inner chamber.
The protective layer is formed from a ceramic coating layer. Or, for example, γ-Al on the surface of the ceramic coating layer₂O_ThreeIt is formed from what provided the layer.
[0004]
In the oxygen sensor element, the exhaust gas is contained in the protective layer (ceramic coating layer, γ-Al₂O_ThreeSensor output can be obtained by passing through the layer) and reaching the measuring electrode.
In addition, the protective layer protects the measurement electrode from being deteriorated by poisoning substances such as P and Pb in the exhaust gas, and has an action of preventing the measurement electrode from peeling from the solid electrolyte body.
[0005]
However, when the operating condition of the internal combustion engine is rich in fuel, that is, when the exhaust gas becomes oxygen lean, the carbon in the carbon compound such as carbon monoxide (CO) and hydrocarbon (HC) in the exhaust gas. Atoms may be carbonized by the catalytic action of the measurement electrode and adhere to the interface between the measurement electrode and the protective layer.
[0006]
For this reason, when the oxygen sensor element is used for a long time, the amount of carbon adhesion increases, and the protective layer cracks or the protective layer peels off. There has been a problem in that the characteristics of these materials are significantly deteriorated.
[0007]
Therefore, conventionally, it has been proposed to provide a dense glassy coating that does not allow exhaust gas to pass through the measurement electrode, particularly in the portion where carbon is likely to adhere (Japanese Patent Laid-Open No. 54-97490).
In this technique, exhaust gas containing a carbon compound can be blocked by the glassy film. For this reason, carbon deposition at the interface between the measurement electrode and the protective layer can be prevented.
Therefore, it is possible to obtain an oxygen sensor element that can maintain good sensor characteristics over a long period of time.
[0008]
[Problems to be solved]
However, the above-described conventional technique has the following problems.
Here, the installation environment of the oxygen sensor element used in combustion control of an automobile internal combustion engine or the like is an environment under severe conditions that are frequently exposed to a cold cycle together with a high temperature atmosphere.
For this reason, each substance constituting the oxygen sensor element is usually required to have a high melting point and an appropriate thermal expansion coefficient.
[0009]
Here, when considering the thermal expansion coefficient of the glassy coating according to the above prior art, this value is 40 to 70 × 10.^-7/ ° C.
However, in this range, the solid electrolyte body constituting the oxygen sensor element (thermal expansion coefficient is 75 to 85 × 10 × 10).^-7Since the difference in coefficient of thermal expansion between the glassy film and the glassy coating film is large, the glassy film may be cracked or peeled off due to thermal stress.
A vitreous film with cracks and delamination cannot exhibit sufficient exhaust gas blocking effect.
[0010]
Furthermore, the surface of the glassy coating is smooth. Therefore, the adhesive force between the glassy film and the protective layer is weak, and the protective layer may be cracked or peeled off during use of the oxygen sensor element. Furthermore, this crack or peeling may reach the protective layer in a portion where the glassy coating is not provided.
A protective layer with cracks or delamination cannot provide sufficient protection against the measuring electrode.
[0011]
Furthermore, in the above-described conventional technology, the thickness of the glassy coating is as thick as 20 to 100 μm, so that a step is formed between the edge of the glassy coating and the surface of the protective layer, and stress is concentrated on the stepped portion. As a result, the protective layer may be cracked or peeled off.
[0012]
In order to solve such a problem, it has been proposed to increase the thickness of the protective layer particularly in the portion where the carbon easily adheres to prevent the passage of carbon compounds and the like (Japanese Utility Model Publication No. 57-30604). Also, a method for capturing the carbon compound with the protective layer (Japanese Patent Laid-Open No. 63-19549), a method for forming a trap layer for capturing the carbon compound on the protective layer (Japanese Patent Laid-Open No. 63-306279), etc. Has been proposed.
[0013]
However, in any of the methods, when the oxygen sensor element is used for a long time, the carbon compound capturing ability is lowered, so that it is difficult to sufficiently prevent carbon deposition.
In particular, gaseous carbon monoxide (CO), hydrocarbon (HC), etc. can easily pass through the protective layer and the trap layer and reach the measuring electrode, so that it is possible to obtain sufficient effects with the conventional technology. It was difficult.
[0014]
In view of the above problems, the present invention is intended to provide an oxygen sensor element and a method for manufacturing the same that can maintain a stable output during long-term use with almost no carbon deposition on the electrode.
[0015]
[Means for solving problems]
  The invention of claim 1 is an oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, wherein at least one of the electrodes is covered with a protective layer,
  And the protective layer is the surface portionOneA dense layer that is densified from the insideAnd
  The dense layer has a porosity of 4% or less and is formed only in the vicinity of the attachment base portion of the oxygen sensor element.This is an oxygen sensor element.
[0016]
As will be described later, the oxygen sensor element comprises a so-called cup-shaped solid electrolyte body and an inner chamber provided in the so-called cup-type solid electrolyte body. On the outer surface of the solid electrolyte body, a measurement electrode that is in contact with the gas to be measured is provided. And an oxygen sensor element having a reference electrode on the inner surface facing the inner chamber.
[0017]
Alternatively, the oxygen sensor element includes a substrate made of a plate-like solid electrolyte body and a pair of measurement electrodes and a reference electrode provided on the substrate, and the measurement electrode is configured to be in contact with the gas to be measured. A stacked oxygen sensor element can be mentioned.
In any configuration, a protective layer that covers the electrode is provided on the electrode that is in contact with the gas to be measured (the measurement electrode corresponds to this in the above-described configuration).
[0018]
The present invention can be applied to oxygen sensor elements having other configurations. Other structures include those provided with electrodes having a pumping action and those having electrodes other than the pair of electrodes described above.
[0019]
Further, the protective layer and the dense layer do not need to cover all of the electrodes, and may be provided only partially. In particular, since the dense layer blocks the gas to be measured, it is preferable that the dense layer be provided so as to avoid the portion of the electrode related to oxygen concentration detection.
[0020]
The operation of the present invention will be described below.
The surface portion of the protective layer of the oxygen sensor element according to the present invention forms a dense layer.
Therefore, it is possible to prevent the gas to be measured from reaching the electrode, and to prevent carbon deposition on the electrode.
Further, in the present invention, since the measurement gas can be prevented from reaching the electrode, it is also possible to particularly prevent the gaseous carbon compound from reaching the electrode.
[0021]
The dense layer according to the present invention is a part of the protective layer.
For this reason, there is almost no difference in coefficient of thermal expansion between the protective layer and the dense layer, and it is possible to prevent peeling of the dense layer, occurrence of cracks, etc. caused by exposure to a thermal cycle or a high temperature atmosphere.
Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output over a long period of use.
In addition, since no other coating or the like exists between the protective layer and the electrode, the protective layer can strongly adhere to the electrode. For this reason, the crack, peeling, etc. of the protective layer can also be prevented.
[0022]
As described above, according to the present invention, it is possible to provide an oxygen sensor element capable of maintaining a stable output during long-term use with almost no carbon deposition on the electrode.
[0023]
In addition, a trap layer can be provided so as to cover the protective layer and the dense layer (see FIG. 6 described later). Thereby, it is possible to prevent the poisonous substance in the measurement gas from reaching the electrode. Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output even when used for a longer period of time.
[0024]
The trap layer is a porous layer formed of a large number of particles and has a function of capturing poisonous substances such as P, Ca, and Pb contained in the gas to be measured.
The trap layer is, for example, γ-Al₂O_ThreeA slurry prepared by adding water, an inorganic binder, and a dispersant to heat-resistant particles such as the above is adhered to these surfaces by dipping or spraying so as to cover the protective layer and the dense layer, and further at 500 to 900 ° C. It can be formed by baking.
In addition, it is preferable that the baking temperature of the said trap layer is lower than the heating temperature at the time of forming the dense layer mentioned later.
[0025]
  Also,The porosity of the dense layer is 4% or lessThe
  Thereby, the effect concerning this invention can be acquired reliably.
  When the porosity is higher than 4%, the gas to be measured is not sufficiently blocked, and the gas to be measured may reach the electrode while diffusing the protective layer through the pores existing in the dense layer. . The porosity is more preferably 3% or less.
[0026]
  Next, the claim2As described above, the dense layer preferably has a porosity of 4% or less and a thickness of 5 to 20 μm.
  Thereby, the effect concerning this invention can be acquired reliably.
  If the porosity is greater than 4%, the gas to be measured may not be shut off as described above.
  When the dense layer has a thickness of less than 5 μm, if the dense layer is too thin and the porosity is high, gas may pass through and the gas blocking effect may not be exhibited. On the other hand, when the thickness is greater than 20 μm, it is difficult to allow the densified material to penetrate into the protective layer, which may make manufacture difficult.
[0027]
  Also,The dense layer is formed near the base of the oxygen sensor element.The
  Since this portion is the portion where carbon is most likely to precipitate as will be described later, the effects of the present invention can be obtained more reliably.
[0028]
Here, the mounting base portion will be described.
In general, an oxygen sensor element is used by being installed in an oxygen sensor constituted by a housing, an atmosphere side cover and the like. When attaching the oxygen sensor element to the oxygen sensor, for example, as shown in FIG. 4 described later, the oxygen sensor element is attached in contact with the housing. In this case, the portion where the oxygen sensor element and the housing are in contact is the mounting base portion.
[0029]
The vicinity of the mounting base portion is clear from the figure, but the oxygen sensor element and the housing are close to each other, so that the flow of the gas to be measured tends to stagnate.
Accordingly, in the vicinity of the attachment root portion, the time of exposure to the carbon compound is longer than that in other portions. For this reason, the electrode in the vicinity of this attachment base portion is a portion where carbon tends to adhere.
[0030]
Furthermore, since the lower end portion of the oxygen sensor element is exposed to the gas to be measured, even if carbon adheres, the carbon is peeled and removed by the flow of the gas to be measured.
However, since the flow of the gas to be measured tends to stagnate in the vicinity of the attachment base portion of the oxygen sensor element, the above-described peeling and removal is unlikely to occur.
Also from the above points, the vicinity of the attachment base portion is a portion where carbon is likely to adhere.
[0031]
Moreover, it is preferable that the upper end of the dense layer is the mounting base portion, and the lower end of the dense layer is 10 mm above the tip of the oxygen sensor element (see FIG. 5).
When the upper end portion is below the attachment base portion, the gas to be measured may enter through the protective layer in the portion where the dense layer does not exist, thereby causing carbon deposition on the electrode.
When the lower end portion is below 10 mm above, the gas to be measured does not easily reach the portion of the oxygen sensor element that has a dominant role in oxygen concentration detection, and the response of the oxygen sensor element May decrease.
[0032]
  Next, the claim3As in the invention, the dense layer preferably contains at least one of Al and Mg phosphates.
  As a result, a dense layer having heat resistance is formed, there is no thermal deterioration in a high-temperature atmosphere, and MgO.Al that forms a protective layer₂ O_Three Since it is chemically bonded to the spinel, a dense layer having a small difference in thermal expansion coefficient from the protective layer is formed, and cracking and peeling of the dense layer due to a cooling cycle during use can be prevented.
[0033]
  Next, the claim4As described above, the dense layer is preferably filled with ceramic fine particles having a particle size of 0.1 μm or less and sintered.
  By using ceramic fine particles, heat resistance is ensured, and since the thermal expansion coefficient with the protective layer is small, cracking and peeling of the dense layer due to the cold cycle during use can be prevented. More preferablyAs in the invention of claim 5,It is preferable to use fine particles having a size of 0.05 μm or less.
  According to a sixth aspect of the present invention, the dense layer is preferably provided between a portion where the protective layer and the housing are in contact with each other and a portion 10 mm above the tip of the oxygen sensor element. Thereby, an oxygen sensor element with high initial response and high durability can be obtained.
[0034]
When the particle size is larger than 0.1 μm, the number of particles filled in the pores of the protective layer is small and the gap between the particles is large. In addition, since the voids after sintering are large, the pores are not sufficiently blocked and the gas to be measured passes through, so that the gas blocking effect may not be exhibited.
[0035]
  Next, the invention of claim 7 is a cup-shaped solid electrolyte body in which the distal end is closed, the proximal end is opened, and an inner chamber is provided therein,
  A measurement electrode formed on at least a part of the outer surface of the front end and the base end of the solid electrolyte body and exposed to the gas to be measured;
  A reference electrode provided on an inner surface facing the inner chamber and provided to face the measurement electrode via the solid electrolyte body;
  The heater is inserted into the inner chamber and heats the tip side of the solid electrolyte body, and the measurement electrode is covered with a protective layer.
  And the surface part of the base end side in the protective layerOnly the porosity is less than 4%The oxygen sensor element is characterized by forming a dense layer (see FIG. 1 described later).
[0036]
The base end side of the protective layer according to the present invention is a dense layer, and the porosity of the dense layer is smaller than that of the protective layer. That is, it is denser than the protective layer.
For this reason, it is possible to prevent the gas to be measured from reaching the measurement electrode in the portion where the dense layer is present, and thus it is possible to prevent carbon from being deposited on the measurement electrode in this portion. In the present invention, an effective effect on a gaseous carbon compound can be obtained.
[0037]
The dense layer according to the present invention is a part of the protective layer.
For this reason, there is almost no difference in coefficient of thermal expansion between the protective layer and the dense layer, and it is possible to prevent peeling of the dense layer, occurrence of cracks, etc. caused by exposure to a thermal cycle or a high temperature atmosphere.
Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output over a long period of use.
Further, since no other film or the like exists between the protective layer and the measurement electrode, the protective layer can strongly adhere to the measurement electrode. For this reason, the crack, peeling, etc. of the protective layer can also be prevented.
[0038]
Further, a heater is inserted into the oxygen sensor element, and the heater is configured to mainly heat the tip end side of the oxygen sensor element. For this reason, even if carbon adheres to the front end side of the oxygen sensor element, the adhering carbon is burned and removed by the heat of the heater.
However, on the base end side of the oxygen sensor element, almost no combustion removal due to the heat of the heater occurs, and carbon adheres easily.
Therefore, the effect of the present invention can be further ensured by providing a dense layer on the base end side.
[0039]
As described above, according to the present invention, it is possible to provide an oxygen sensor element capable of maintaining a stable output during long-term use with almost no carbon deposition on the electrode.
[0040]
The base end side indicates the side closer to the base end of the oxygen sensor element in the protective layer, as shown in FIG. Similarly, the tip portion side also shows the side closer to the tip portion of the oxygen sensor element in the protective layer.
[0041]
  Also,The porosity of the portion formed on the base end side of the protective layer is 4% or lessThe
  Thereby, the effect concerning this invention can be acquired reliably.
  If the porosity is greater than 4%,In the case of the invention of claim 1Like the case, there is a possibility that the gas to be measured cannot be shut off.
[0042]
  Next, the claim8As in the invention, the thickness of the part formed on the base end side of the protective layer is preferably 5 to 20 μm.
  Thereby, the effect concerning this invention can be acquired reliably.
  When the thickness of the dense layer is less than 5 μm,In the case of the invention of claim 1Similarly, the dense layer is too thin and the gas may pass therethrough, and the gas blocking effect may not be exhibited. On the other hand, if the thickness is greater than 20 μm, as in claim 2, it is difficult to allow the densified material to penetrate into the protective layer, which may make manufacturing difficult.
[0043]
  Next, the claim9In the oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, at least one of the electrodes is covered with a protective layer,
  And the protective layerOn the proximal sideSurface partOnlyDense from insidePorosity of 4% or lessA method of manufacturing an oxygen sensor element in which a dense layer is formed,
  The oxygen sensor element manufacturing method is characterized in that the dense layer is formed by applying a substance reacting with the protective layer material constituting the protective layer to the protective layer, impregnating and then heating.
[0044]
The heating temperature is preferably 500 to 1000 ° C.
When the heating temperature is less than 500 ° C., the thermal history when using the oxygen sensor element may exceed the heating temperature, and the reaction between the protective layer material and the substance is in use of the oxygen sensor element. This may cause cracks in the protective layer.
[0045]
On the other hand, when the heating temperature exceeds 1000 ° C., the heating temperature at the time of electrode preparation may be exceeded, and there is a possibility that aggregation or the like occurs in the electrode. Since the electrode in which the aggregation has occurred decreases the catalyst performance, the performance of the oxygen sensor element may also decrease.
[0046]
In producing the oxygen sensor element, after producing a solid electrolyte body, an electrode is produced thereon, and a protective layer is produced so as to cover the electrode. Thereafter, a dense layer can be formed on the protective layer.
[0047]
In the present invention, when the substance is applied and heated, the substance reacts with the protective layer material to form a compound. At this time, the compound expands in volume and can close pores near the surface of the protective layer (see FIG. 3).
[0048]
Therefore, according to the manufacturing method of the present invention, a part of the surface of the protective layer forms a dense layer, and therefore, as shown in claim 1, the difference in thermal expansion coefficient between the protective layer and the dense layer. Thus, it is possible to obtain an oxygen sensor element that is almost free from the occurrence of cracks and the like due to the release of a dense layer and exposure to a high temperature atmosphere.
Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output over a long period of use.
[0049]
In addition, since no other coating or the like exists between the protective layer and the electrode, the protective layer can strongly adhere to the electrode. For this reason, the crack, peeling, etc. of the protective layer can also be prevented.
Since the dense layer can prevent the gas to be measured from reaching the electrode, it is possible to obtain an oxygen sensor element with almost no carbon deposition on the electrode.
[0050]
As described above, according to the present invention, it is possible to provide a method for producing an oxygen sensor element that can maintain a stable output in a long-term use with almost no carbon deposition on the electrode.
[0051]
The type of substance used to produce the dense layer depends on the type of protective layer material, but it reacts with the constituent material of the protective layer and forms a thermally and chemically stable compound after the reaction. Preferably, for example, the protective layer is MgO · Al₂O_ThreeIn the case of spinel, it is preferable to use phosphoric acid.
As a result, Mg, Al and phosphoric acid forming the protective layer react with each other to form phosphate, and further volume expansion causes the pores of the protective layer to be blocked, thereby blocking the gas to be measured.
[0052]
In preparing the dense layer, the substance is made into a liquid or double diluted solution, which is applied to a desired portion of the protective layer using a method such as brush painting or dipping, and thereafter. It is preferable to heat this.
Further, it is preferable to mask in advance a portion where application is not required, that is, a portion where the dense layer is unnecessary.
[0053]
Alternatively, a 5 to 10-fold diluted solution of the above substance is prepared, applied to the protective layer, and heated. Further, the above 5 to 10-fold diluted solution is applied and heated repeatedly. The dense layer can also be produced by the method as described above.
[0054]
  Claims10In the oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, at least one of the electrodes is covered with a protective layer,
  And the protective layerOn the proximal sideSurface partOnlyDense from insidePorosity of 4% or lessA method of manufacturing an oxygen sensor element in which a dense layer is formed,
  The oxygen sensor element manufacturing method is characterized in that the fine pores in the protective layer are filled with ceramic fine particles having a particle size of 0.1 μm or less, and the fine particles are sintered to form the dense layer. .
  More preferably, the particle diameter of the fine particles is 0.05 μm or less.
[0055]
The fine particles are preferably made of a material that is the same as the protective layer, or a material that is the same as the protective layer after sintering. As the fine particles, for example, heat-resistant particles such as alumina and silica, or particles that can acquire heat resistance after sintering can be used.
[0056]
These fine particles are in a sol state, and an aqueous solution containing this sol is applied to the surface of the protective layer by brushing, dipping, etc., and impregnated to fill the pores of the protective layer with the fine particles. It is preferable.
The fine particles are preferably attached in a reduced pressure atmosphere. Thereby, the fine particles can be filled into the pores.
[0057]
If the particle size (diameter) of the fine particles is larger than 0.1 μm, the number of fine particles filled in the pores of the protective layer is reduced, and the pores are not sufficiently blocked, and the measurement target There is a possibility that a dense layer that cannot sufficiently block the gas is obtained.
[0058]
The sintering is preferably performed by heating at a temperature of 500 to 1000 ° C.
If the temperature is lower than 500 ° C., the thermal history when using the oxygen sensor element may exceed the above temperature, so that the sintering of the fine particles may occur during the use of the oxygen sensor element and the protective layer may crack. May occur.
[0059]
On the other hand, when the temperature exceeds 1000 ° C., the heating temperature at the time of electrode production may be exceeded, and aggregation or the like may occur in the electrode. Since the electrode in which the aggregation has occurred decreases the catalyst performance, the performance of the oxygen sensor element may also decrease.
[0060]
In the present invention, the pores in the protective layer are filled with ceramic fine particles having a particle size of 0.1 μm or less, and these fine particles are sintered. As a result, the fine particles are bonded to each other, and can be further bonded to the protective layer material constituting the protective layer. For this reason, pores near the surface of the protective layer can be blocked (see FIGS. 7 and 8).
More preferably, the particle diameter of the fine particles is 0.05 μm or less.
[0061]
Therefore, according to the manufacturing method of the present invention, since a part of the surface of the protective layer forms a dense layer, as shown in claim 1, there is a difference in thermal expansion coefficient between the protective layer and the dense layer. It is possible to obtain an oxygen sensor element that is almost free from the occurrence of cracking, cracking, and the like of a dense layer due to a cold cycle and exposure to a high temperature atmosphere.
Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output over a long period of use.
[0062]
In addition, since no other coating or the like exists between the protective layer and the electrode, the protective layer can strongly adhere to the electrode. For this reason, the crack, peeling, etc. of the protective layer can also be prevented.
Since the dense layer can prevent the gas to be measured from reaching the electrode, it is possible to obtain an oxygen sensor element with almost no carbon deposition on the electrode.
[0063]
As described above, according to the present invention, it is possible to provide a method for producing an oxygen sensor element that can maintain a stable output in a long-term use with almost no carbon deposition on the electrode.
[0064]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1
An oxygen sensor element according to an embodiment of the present invention and a manufacturing method thereof will be described with reference to FIGS.
As shown in FIGS. 1 and 2, the oxygen sensor element 1 according to this example includes a solid electrolyte body 10, a pair of measurement electrodes 11 and a reference electrode 14 provided on the surface of the solid electrolyte body 10.
The measurement electrode 11 is covered with a protective layer 12, and the protective layer 12 forms a dense layer 13 that is densified from the inside in part of the surface portion.
[0065]
The structure of the oxygen sensor element 1 will be described in detail.
As shown in FIG. 1, the oxygen sensor element 1 comprises a cup-shaped solid electrolyte body 10 with a distal end portion 191 closed and a proximal end portion 192 opened, and an inner chamber 19 provided therein, and the solid state solid body. The measurement electrode 11 is provided on the outer surface 101 of the electrolyte body 10, and the reference electrode 14 is provided on the inner surface 109 facing the inner chamber 19. A heater (not shown) is inserted into the inner chamber 19, and the measuring electrode 11 is provided with a protective layer 12 for protecting it.
[0066]
The protective layer 12 is MgO.Al.₂O_ThreeMade of spinel. As shown in FIG. 3, the protective layer 12 is a porous layer formed by plasma spraying. Reference numeral 129 denotes MgO · Al.₂O_ThreeSpinel particles.
The dense layer 13 on the surface of the protective layer 12 is formed so as to close the pores existing on the surface of the porous protective layer 12, and contains aluminum phosphate or the like.
[0067]
A convex portion 105 is formed on the outer periphery of the solid electrolyte body 10, and the convex portion 105 has a tapered portion 108. The protective layer 12 and the dense layer 13 are also formed on the tapered portion 108.
In the taper portion 108, the oxygen sensor element 1 according to this example is attached to the housing 30 of the oxygen sensor 3 described later (see FIG. 4). That is, the taper portion 108 becomes a mounting base portion of the oxygen sensor element 1.
[0068]
Next, the oxygen sensor 3 to which the oxygen sensor element 1 is attached will be described.
As shown in FIG. 4, the oxygen sensor 3 includes a housing 30 and an oxygen sensor element 1 inserted in the housing 30. A measured gas chamber 33 is formed below the housing 30, and a double measured gas side cover 330 for protecting the tip of the oxygen sensor element 1 is provided. The measurement gas side cover is provided with an introduction hole 331 for introducing the measurement gas.
[0069]
Further, a rod-shaped heater 34 is inserted into the inner chamber 19 of the oxygen sensor element 1. The heater 34 is inserted and arranged with a desired clearance between the inner surface 109 of the inner chamber 19.
[0070]
Above the housing 30, three stages of atmospheric side covers 31, 32, 33 are provided. An elastic insulating member 39 through which lead wires 391 to 393 are inserted is provided at the upper ends of the atmosphere side covers 32 and 33. The lead wires 391 and 392 take out the current generated in the solid electrolyte body 2 as a signal and send it to the outside. The lead wire 393 is for energizing the heater 34 to generate heat.
[0071]
Connection terminals 383 and 384 are provided at the lower ends of the lead wires 391 and 392. The connection terminals 383 and 384 are connected to the terminals 381 and 382 fixed to the oxygen sensor element 1.
The terminals 381 and 382 are fixed in contact with the ends of the measurement electrode and the reference electrode in the oxygen sensor element 1.
[0072]
Next, the manufacturing method of the oxygen sensor element 1 will be described in detail.
ZrO₂A raw material powder such as the above is weighed and mixed to produce a cup-shaped molded body as shown in FIG. Next, the molded body is fired to form the solid electrolyte body 10.
Next, Pt is attached to the outer surface 101 and the inner surface 109 of the solid electrolyte body 10 using any one of chemical plating, vapor deposition, paste coating, baking, and the like to form the measurement electrode 11 and the reference electrode 14. To do.
[0073]
Next, MgO · Al is formed on the surface of the measurement electrode 11.₂O_ThreeThe protective layer 12 is formed by plasma spraying spinel. The protective layer 12 has a thickness of 50 to 300 μm, a porosity of 5 to 30%, and an average pore diameter of 500 to 5000 mm.
Next, 100% phosphoric acid is applied to the surface of the protective layer 12 by brush painting.
Thereafter, the solid electrolyte body 10 is heated to 900 ° C.
[0074]
By this heating, the phosphoric acid and MgO.Al in the protective layer 12₂O_ThreeSpinel reacts to form aluminum phosphate and the like. Furthermore, when the aluminum phosphate or the like expands in volume, the pores on the surface portion of the protective layer 12 are closed. As a result, the porosity of the surface of the protective layer 12 is about 3%, and the dense layer 13 that does not allow the gas to be measured to pass through is formed. The dense layer has a thickness of 10 μm.
Thus, the oxygen sensor element 1 according to this example was obtained.
[0075]
Next, the performance of the oxygen sensor element according to this example will be described with reference to FIG.
As for the oxygen sensor element according to this example, Samples 1 to 13 manufactured by varying the position where the dense layer was provided and the thickness T of the dense layer as shown in Table 1 were prepared. The durability and initial response of these samples were measured as shown below.
[0076]
First, the position where the dense layer is provided will be described.
F in the table is a distance between the oxygen sensor element-housing contact position and the densification start position (the upper end of the dense layer) as shown in FIG. The contact position between the oxygen sensor element and the housing is the attachment base portion in this example.
[0077]
In addition, G in the table is the distance between the densification end position (the dense layer lower end) and the oxygen sensor element tip, as shown in FIG.
And the oxygen sensor element which changed F in the range of 5-5 mm and G in the range of 0-10 mm was produced.
Note that “−” in F indicates that the dense layer is provided up to the position facing the inside of the housing beyond the contact position of the oxygen sensor element-housing.
Further, an oxygen sensor element was produced in which the thickness T of the dense layer was changed in the range of 2 to 20 μm. The thickness T of the dense layer is illustrated in FIG.
[0078]
Next, the performance of the oxygen sensor element was shown by durability and initial response as shown in Table 1.
The durability was determined by the adhesion and appearance of the protective layer after the accelerated durability test.
In the accelerated durability test, the oxygen sensor according to FIG. 4 in which the oxygen sensor element as each sample was attached to the exhaust system of a 2000 cc in-line four-cylinder engine with a fuel injection device was arranged. The exhaust gas temperature was set to 850 ° C. and the excess air ratio in the exhaust gas was set to 0.90 (reducing atmosphere). The durability time was 100 hours.
[0079]
After the accelerated durability test, taping evaluation was performed on the protective layer of the oxygen sensor element applied to each sample. In this evaluation, the case where the protective layer was peeled off was judged as x, and the case where the protective layer was not peeled off was judged as ◯, and it was described in the column of adhesive force in Table 1.
The appearance of the protective layer on the oxygen sensor element on each sample was observed with the naked eye to confirm the presence or absence of cracks and peeling. The case where the protective layer had cracks or peeling was judged as “X”, and the case where there was no crack or peeling was judged as “◯”, and was described in the column of appearance.
[0080]
The initial response was measured as follows.
The oxygen sensor according to FIG. 4 in which an oxygen sensor element as each sample was attached to the exhaust system of a 2000 cc in-line 6-cylinder engine with a fuel injection device was arranged.
[0081]
And, using unleaded gasoline, the rotation speed is 1100 r. p. m. The engine was operated and the tip temperature of the oxygen sensor element was maintained at 690-710 ° C. Then, the time for the output of the oxygen sensor element to change from 0.6V to 0.3V at the time of switching from λ = 0.9 to λ = 1.1 was measured.
The case where this time is less than 150 ms is judged as ◯, the case where it is 150 ms or more and less than 200 ms is judged as Δ, and the case where it is 200 ms or more is judged as ×.
The above measurement was performed before and after the durability test.
[0082]
By providing at least a dense layer between the portion where the protective layer and the housing are in contact with each other and the portion 10 mm above the tip of the oxygen sensor element by using Sample 3 to Sample 9, Sample 12 and 13 in the table, It was found that an oxygen sensor element with high initial response and high durability was obtained.
[0083]
Next, the effect in this example is demonstrated.
The surface portion of the protective layer 12 of the oxygen sensor element 1 according to the present example forms a dense layer 13, thereby preventing the measurement gas from reaching the measurement electrode 11. Therefore, it is possible to prevent carbon from being deposited on the measurement electrode 11.
[0084]
Further, the dense layer 13 according to this example is a part of the protective layer 12.
For this reason, there is almost no difference in thermal expansion coefficient between the protective layer 12 and the dense layer 13, and thermal stress is generated between the two due to exposure to a cooling cycle and a high temperature atmosphere, and the dense layer 13 is peeled off. , Cracks can be prevented.
Therefore, it is possible to obtain the oxygen sensor element 1 that can maintain a stable output during long-term use.
[0085]
In addition, since no other film or the like exists between the protective layer 12 and the measurement electrode 11, the protective layer 12 can strongly adhere to the measurement electrode 11. For this reason, the crack, peeling, etc. of the protective layer 12 can also be prevented.
[0086]
As described above, according to this example, it is possible to provide an oxygen sensor element capable of maintaining a stable output during long-term use with almost no carbon deposition on the electrode, and a method for manufacturing the oxygen sensor element.
Further, according to the manufacturing method of this example, an excellent oxygen sensor element as described above can be manufactured.
[0087]
Further, in the present example, the dense layer 13 is formed in a tapered portion 108 which is a mounting base portion of the oxygen sensor element 1.
Since this portion is the portion where carbon is most likely to precipitate as will be described later, the effect of this example can be obtained more reliably.
[0088]
That is, a heater 34 is inserted into the oxygen sensor element 1, and the heater 34 is configured to mainly heat the vicinity of the tip 101 of the oxygen sensor element 1. For this reason, even if carbon adheres in the vicinity of the tip portion 101 of the oxygen sensor element 1, the adhering carbon is burned and removed by the heat of the heater 34. However, almost no combustion removal due to the heat of the heater 34 occurs in the mounting base portion.
[0089]
Furthermore, as shown in FIG. 4, the tip 101 of the oxygen sensor element 1 faces an introduction hole 331 for introducing the gas to be measured into the gas chamber 33 to be measured. It is removed by the flow of the measurement gas.
However, since the flow of the gas to be measured tends to stagnate at the attachment base portion of the oxygen sensor element 1, the above-described peeling and removal is hardly caused.
[0090]
As shown in FIG. 6, a trap layer 15 can be provided outside the protective layer 12 and the dense layer 13.
Thereby, the poisonous substance in the measurement gas can be prevented from reaching the measurement electrode 11. Therefore, it is possible to obtain the oxygen sensor element 1 that can maintain a stable output even when used for a longer period of time.
[0091]
[Table 1]

[0092]
Embodiment 2
In this example, an oxygen sensor element having a dense layer prepared by filling pores in a protective layer with ceramic fine particles having a particle size of 0.1 μm or less and sintering these fine particles, and production of the dense layer according to this example The method will be described with reference to FIGS.
[0093]
The oxygen sensor element according to this example has the same structure as that of the first embodiment, and includes a cup-shaped solid electrolyte body 10, a pair of measurement electrodes 11 provided on the surface of the solid electrolyte body 10, a reference electrode, It becomes more.
The measurement electrode 11 is covered with a protective layer 12, and the protective layer 12 forms a dense layer 13 that is densified from the inside in part of the surface portion.
The solid electrolyte body 10 is provided with an inner chamber 19 into which a heater (not shown) is inserted.
[0094]
The protective layer 12 and the dense layer 13 according to this example will be described.
The protective layer 12 according to this example is also MgO · Al as in the first embodiment.₂O_ThreeMade of spinel. Further, as shown in FIG. 7, the protective layer 12 is a porous layer formed by plasma spraying. Reference numeral 129 denotes MgO · Al.₂O_ThreeSpinel particles.
And the pore in the surface of the said protective layer 12 is obstruct | occluded by many fine particles 139, and the dense layer 13 will be formed here.
The fine particles 139 are made of Al.₂O_ThreeIt consists of particles, and its particle size is 0.1 μm or less. The dense layer 13 has a porosity of 3%.
Others are the same as those of the oxygen sensor element of the first embodiment.
[0095]
A method for manufacturing the dense layer according to this example will be described below.
In the same manner as in the first embodiment, the solid electrolyte body 10, the measurement electrode 11, the protective layer 12, and the like are produced.
Next, an alumina sol solution was applied to the protective layer 12 by brush painting. As a result, alumina fine particles 139 present in the alumina sol were filled in the pores on the surface of the protective layer 12.
[0096]
Next, the coated alumina sol solution together with the solid electrolyte body is heated. Thereby, the fine particles 139 were sintered with each other, or the fine particles 139 and the particles 129 constituting the protective layer 12 were sintered to form the dense layer 13.
[0097]
Next, the performance of the oxygen sensor element according to this example will be described with reference to FIG.
Regarding the oxygen sensor element according to this example, samples 21 to 29 manufactured by changing the position where the dense layer 13 was provided as shown in Table 2 were prepared. The durability and initial response of these samples were measured in the same manner as in Example 1. In the table, F and G have the same meanings as in the first embodiment, table 1, and FIG.
[0098]
According to Sample 23 to Sample 25 and Samples 28 and 29 in Table 2, as in the first embodiment, a portion where the protective layer and the housing for the oxygen sensor are in contact with each other, and a portion 10 mm above the tip of the oxygen sensor element. It was found that an oxygen sensor element with high initial response and high durability was obtained by providing at least a dense layer between them. It was also found that the oxygen sensor element excellent in the manufacturing method according to the present example can be manufactured as in the first embodiment.
[0099]
[Table 2]

[0100]
Embodiment 3
In this example, as shown in FIG. 9, an oxygen sensor element having a dense layer made of glass is described.
The oxygen sensor element according to this example has the same structure as that of the first embodiment, and includes a cup-shaped solid electrolyte body 10, a pair of measurement electrodes 11 provided on the surface of the solid electrolyte body 10, a reference electrode, It becomes more.
The measurement electrode 11 is covered with a protective layer 12, and the protective layer 12 forms a dense layer 13 that is densified from the inside in part of the surface portion.
The solid electrolyte body 10 is provided with an inner chamber, and a heater (not shown) is inserted into the inner chamber.
[0101]
The dense layer will be described.
The dense layer 13 according to this example is made of a glassy heat-resistant film.
Specifically, MgO and Al₂O_ThreeAnd a solution obtained by dissolving a resin such as ethyl cellulose in a low melting point glass such as borosilicate glass is made into a slip shape, and this is attached to a desired portion of the protective layer 12 by brush coating, and a temperature of 800 to 1000 ° C. It consists of a film formed by baking.
[0102]
When the temperature is less than 800 ° C., the glass in the material is not sufficiently melted, the adhesion with the protective layer is insufficient, and the dense layer may be peeled off during use of the oxygen sensor element. On the other hand, when the said temperature is higher than 1000 degreeC, there exists a possibility that an electrode may aggregate.
[0103]
The thermal expansion coefficient of the dense layer 13 according to this example has an expansion coefficient substantially equal to that of the protective layer. That is, the thermal expansion coefficient of the protective layer 12 is 75 to 85 × 10.^-7/ ° C., the expansion coefficient of the dense layer 13 is 75 to 85 × 10^-7/ ° C.
[0104]
In addition, as the above-mentioned film, if the thermal expansion coefficient is the same as or close to that of the protective layer 12, and is a material that does not soften at a temperature of about 500 to 700 ° C. that is the use temperature of the oxygen sensor element, other than the above-described materials You may be comprised with the substance.
[0105]
Next, the performance of the oxygen sensor element according to this example will be described with reference to FIGS.
That is, an oxygen sensor element (sample 31) provided with a dense layer such that F and G according to FIG. 5 have values shown in Table 3 was prepared, and durability and initial response were measured in the same manner as in the first embodiment. did.
As a result, as is known from the table, it was found that the oxygen sensor element having the dense layer according to this example had excellent durability and initial response.
[0106]
[Table 3]

[0107]
Embodiment 4
This example is a limiting current type oxygen sensor element 4 as shown in FIG.
The oxygen sensor element 4 according to this example includes a cup-shaped solid electrolyte body 10 and an inner chamber 19 provided in the cup-type solid electrolyte body 10. The reference electrode 14 is provided on the inner surface facing the inner chamber 19.
In addition, a heater (not shown) is inserted in the inner chamber 19, protective layers 43 and 422 are provided on the measuring electrode 421, and a poison trap layer 423 is provided on the outer side. .
[0108]
The protective layer 43 is made of MgO · Al.₂O_ThreeIt is a porous layer made of spinel and formed by plasma spraying. The dense layer 13 on the surface of the protective layer 43 is formed so as to close the pores existing on the surface of the porous protective layer 43, and contains aluminum phosphate.
The dense layer 13 is produced by applying phosphoric acid to the surface of the protective layer 43 by brushing and heating it as shown in the first embodiment.
Others are the same as the first embodiment.
Further, the oxygen sensor element 4 of this example has the same effect as that of the first embodiment.
[Brief description of the drawings]
FIG. 1 is a longitudinal cross-sectional explanatory view of an oxygen sensor element according to Embodiment 1;
2 is a cross-sectional view of the oxygen sensor element taken along the line AA in FIG. 1 according to the first embodiment.
FIG. 3 is an enlarged explanatory view of main parts of an oxygen sensor element according to Embodiment 1;
FIG. 4 is a cross-sectional explanatory view of an oxygen sensor provided with an oxygen sensor element according to this example according to Embodiment 1;
5 is an explanatory diagram showing F and G in Table 1 according to Embodiment 1. FIG.
6 is a longitudinal cross-sectional explanatory view of an oxygen sensor element provided with a trap layer according to Embodiment 1. FIG.
FIG. 7 is an enlarged explanatory view of a main part of an oxygen sensor element according to a second embodiment.
FIG. 8 is an explanatory diagram of a protective layer and a dense layer according to the second embodiment.
FIG. 9 is an enlarged explanatory view of a main part of an oxygen sensor element according to a third embodiment.
10 is a longitudinal cross-sectional explanatory view of an oxygen sensor element according to Embodiment 4. FIG.
[Explanation of symbols]
1,4. . . Oxygen sensor element,
10. . . Solid electrolyte body,
101. . . Outer surface,
109. . . Inner surface,
11. . . Measuring electrode,
12, 43. . . Protective layer,
13. . . Dense layer,
14 . . Reference electrode,
191. . . Tip,
192. . . Proximal end,

Claims (10)

In the oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, at least one of the electrodes is covered with a protective layer,
And the protective layer forms a dense layer that is densified more interior part of the surface portion,
The oxygen sensor element, wherein the dense layer has a porosity of 4% or less, and is formed only in the vicinity of a mounting base portion of the oxygen sensor element. 2. The oxygen sensor element according to claim 1, wherein the dense layer has a porosity of 4% or less and a thickness of 5 to 20 [mu] m. 3. The oxygen sensor element according to claim 1, wherein the dense layer contains a phosphate of at least one of Al and Mg. 4. The oxygen sensor element according to claim 1, wherein the dense layer is filled with ceramic fine particles having a particle diameter of 0.1 μm or less and sintered. 5. The oxygen sensor element according to claim 1, wherein the dense layer is filled with ceramic fine particles having a particle diameter of 0.1 μm or less. 6. The dense layer according to claim 1, wherein the dense layer is provided between a portion where the protective layer and the housing are in contact with each other and a portion 10 mm above the tip of the oxygen sensor element. Oxygen sensor element. A cup-shaped solid electrolyte body in which the distal end is closed, the proximal end is opened, and an inner chamber is provided inside;
A measurement electrode formed on at least a part of the outer surface of the front end and the base end of the solid electrolyte body and exposed to the gas to be measured;
A reference electrode provided on an inner surface facing the inner chamber and provided to face the measurement electrode via the solid electrolyte body;
The heater is inserted into the inner chamber and heats the tip side of the solid electrolyte body, and the measurement electrode is covered with a protective layer.
An oxygen sensor element, wherein a dense layer having a porosity of 4% or less is formed only on the surface of the protective layer on the base end side.   8. The oxygen sensor element according to claim 7, wherein the dense layer has a thickness of 5 to 20 [mu] m. In the oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, at least one of the electrodes is covered with a protective layer,
And a method of manufacturing an oxygen sensor element in which a dense layer having a porosity of 4% or less, which is densified from the inside , is formed only on the surface portion on the base end side of the protective layer,
A method for producing an oxygen sensor element, wherein a substance reacting with a protective layer material constituting the protective layer is applied to the protective layer, impregnated, and then heated to form the dense layer. In the oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, at least one of the electrodes is covered with a protective layer,
And a method of manufacturing an oxygen sensor element in which a dense layer having a porosity of 4% or less, which is densified from the inside , is formed only on the surface portion on the base end side of the protective layer,
A method for producing an oxygen sensor element, wherein the pores in the protective layer are filled with ceramic fine particles having a particle size of 0.1 μm or less, and the fine particles are sintered to form the dense layer.

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